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Hi would someone be able to assist with the following question? The question is from a past paper in preparation for an exam.

Consider a router that interconnects three subnets: Subnet 1, Subnet 2, Subnet 3. Suppose all of the interfaces in each of these subnets are required to have the prefix 223.1.17/24. Also suppose that subnet 1 is required to support up to 125 different hosts, and subnets 2 and 3 are each required to support up to 60 different hosts.

Provide three network addresses (of the form a.b.c.d/x) that define the beginning of the IP address range for each subnet, and explain your reasoning.

I think the answer is the following, but I'm not sure.

Subnet 1: 223.1.17.1/25

Subnet 2: 223.1.17.128/26

Subnet 3: 223.1.17.193/26

Regards.

Community
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siu07
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  • The answer is correct. You can use an online subnet calculator (for example, http://www.subnet-calculator.com/) in order to verify the results for this kind of question. – jmpcm May 03 '11 at 14:37
  • Can you explain how you got to this answer? Quickly skimming over it, I only see one possible mistake that's related to how the question is worded ("that define the beginning of the IP address range"). – onitake May 03 '11 at 14:42

2 Answers2

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Not quite, Network addresses are always the first Addresses in a Subnet, so the answers would be:

223.1.17.0/25, beginning of IP Address Range: 223.1.17.1 (until .126)

223.1.17.128/26, beginning of IP Address Range: 223.1.17.129 (until .190)

222.1.17.192/26, beginning of IP Address Range: 223.1.17.193 (until .254)

Other than that, your CIDR-Subnet Length is correct, 1 needs at least 126 Hosts (- BC and NA), which justifies /25 -> 24 bits for Class C, 1 bit for Subnet and 7 bit(=2^7 = 128 - Broadcast - Network Address = 126) for hosts, No. 2 and 3 need at least 62 Hosts (-BC and NA) each.

Dennis Röttger
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  • Hi thanks for your answer. I just watched a video that says a subnet cannot contain only one subnet bit because that breaks the rule "All subnet bits cannot be all ON or OFF" Our answer to subnet 1 only has 1 subnet bit. Is this answer still valid? It seemed good to me until I watched this http://www.youtube.com/watch?v=6XlqzXyRihY&feature=related. – siu07 May 04 '11 at 10:04
  • Actually that doesn't strike me as entirely true, yes, a Class C Net usually requires a Network-Address and a Broadcast-Address itself, thus disabling the first and the last subnet for their need of the IPs x.x.x.0 and x.x.x.255, so if you only have 1 subnet-bit (=2 subnets), those would essentially not function, however, this is archaic since RFC1812 defined specifications for routers to ignore exactly this behaviour, enabling them again and making 126-host-subnets in a Class-C net possible again. – Dennis Röttger May 04 '11 at 10:10
  • It strikes me that this probably didn't even refer to your question (even though that might also help you understand subnets), but essentially you now got 25 Subnet-Bits in your first Subnet, CIDR means Classless (!) Inter-Domain Routing, so we don't care much for the class C and simply know that we have 25 Subnet-Bits and 7 Hostbits. – Dennis Röttger May 04 '11 at 10:17
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You can see that by using a IP calculator. There are many on the net that also show you some more details..

My favorite (including IPv6) is at http://netools.ch

Simon
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